1. Technical Field
This disclosure relates to facsimile communication systems, network facsimile apparatuses that conform to ITU-T Recommendation T.38 and can efficiently transport packets, and methods thereof.
2. Description of the Related Art
ITU-T Recommendations T.37 and T.38 are protocols for sending a facsimile signal over an IP network.
ITU-T Recommendation T.37 employs “Store and Forward” that temporarily spools facsimile data on an FOIP gateway. That is, facsimile data are temporarily stored in the memory of a gateway as TIFF image data, for example. Communication in the network layer (between gateways) is performed by another protocol. In a case where a mail transfer protocol (SMTP) is used for communication in the network layer, problems occur: the communication lacks real-time characteristics, negotiation among terminals (FAX machines) cannot be established, notification cannot be made to a network facsimile apparatus that is a transmitting source (sender) when transmission fails, and so on. On the other hand, ITU-T Recommendation T.38 is a real-time protocol and capable of causing terminals to directly exchange (transmit/receive) facsimile data between the terminals.
A facsimile signal may be sent over a VoIP (Voice over IP) network as an audio signal. VoIP is a technology of dividing voice into packets and transmitting the packets via a network, and uses an IP address as the address of the packets. There are two protocols, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol), immediately above the IP level (network layer). In voice information transfer using VoIP, generally, data are transferred by using UDP/IP with the objective of ensuring real-time characteristics.
Basically, UDP only adds information relating to the application of a transmission destination (receiver) to the IP protocol, thus lacking reliability. Also, since UDP does not return a response at a protocol level, whether data are received at the transmission destination is unknown, and when an error occurs, the packet is simply discarded. When the transmission destination does not respond for four seconds, for example, a VoIP gateway proceeds to a retransmission process.
On the other hand, when TCP is used, the retransmission procedure is entrusted to a network. When facsimile data are sent using VoIP, however, delay in a data transfer process on the order of several seconds is not a problem. Accordingly, when VoIP is used for facsimile transfer, TCP is used together with VoIP.
By the way, TCP is a higher-level protocol (protocol having higher reliability in data transfer) than UDP. The process work load of TCP, however, is heavy since a retransmission process for a lost packet is performed after time out of the transmitting source, for example. Consequently, if the network traffic is increased, the packet transmission rate may be greatly reduced. Thus, when communication is established with a G3FAX (hereinafter referred to as a “GIIIFAX”) via the VoIP gateway, there is a high probability that a T.30 timer times out due to delay.
On the other hand, in UDP, packet loss is avoided by adding redundant packets. Hence, since the packet size of (number of packets in) a transmission unit becomes large, there is a possibility that a receiving process of a terminal that receives packets is not completed in time.